1. Field of the Invention
The present invention relates to antennas used for mobile communication apparatuses and to radio communication apparatuses including the antennas.
2. Description of the Related Art
In terms of miniaturization, frequency adjustment can be easily achieved, and surface mount antennas are often used for mobile communication apparatuses. In such surface mount antennas, a radiation electrode is provided on a surface of a dielectric substrate to define an inductor, and an open end of the radiation electrode is spaced from a feed electrode so as to define a capacitor. Thus, an LC resonant circuit is provided. High-frequency signals are supplied to the radiation electrode via the feed electrode, thus enabling high-frequency radio transmission.
In accordance with a reduction in the size and an increase in the mounting density of mobile communication apparatuses, in particular, such as cellular telephones, in recent years, more compact surface mount antennas with improved antenna efficiency and a wider bandwidth have been suggested, for example, in Japanese Unexamined Patent Application Publication Nos. 10-173425 and 11-312919.
In addition, recently, in accordance with not only the reduction in the size of antennas but also an increase in the number of functions of cellular telephones, antennas capable of multiband transmission and reception have become available, as described in Japanese Unexamined Patent Application Publication Nos. 2002-158529 and 2002-76750.
In other words, in the antenna described in Japanese Unexamined Patent Application Publication No. 2002-158529, as shown in FIG. 23, a radiation electrode 101 is arranged in a loop on a dielectric substrate 100, and an open end 101a of the radiation electrode 101 faces a feed electrode 102 with a predetermined distance therebetween. Thus, a capacitor is formed between the open end 101a and the feed electrode 102. Changing the capacitance of the capacitor enables multiband performance using a basic mode and a higher mode of the radiation electrode 101, increases the bandwidth, and miniaturizes the antenna.
In the antenna described in Japanese Unexamined Patent Application Publication No. 2002-76750, as shown in FIG. 24, a lumped-constant LC parallel resonant circuit 111 is connected in series to a feeding side of an antenna conductor 110. The antenna conductor 110 is adjusted to resonate at a frequency that is slightly less than a center frequency of an upper frequency band of two frequency bands for transmission and reception. The LC parallel resonant circuit 111 is adjusted to resonate at approximately the center frequency of a lower frequency band for transmission and reception and to provide the antenna conductor 110 with a capacitance to cause the antenna conductor 110 to resonate at the center frequency of the upper frequency band.
However, the known antennas have the following problems.
If the size of the multiband antenna described in Japanese Unexamined Patent Application Publication No. 2002-158529 is microminiaturized to equal to or less than about 1/10 wavelength, the loop diameter of the radiation electrode 101 is reduced. Thus, the capacitance of the capacitor formed by the open end 101a and the feed electrode 102 is increased, and an unwanted capacitance occurs between the loop portion of the radiation electrode 101 and the open end 101a. This causes a reduction in the transmission and reception bandwidth of the antenna and a reduction in the antenna efficiency. Thus, in practice, it is difficult to microminiaturize the antenna. Even if the size of the antenna is maintained large enough not to reduce the bandwidth and not to reduce the antenna efficiency, there is not enough space to add a lumped-constant element, such as an inductor, to the antenna in order to improve the performance of the antenna. Thus, there is very little flexibility in designing the antenna to improve the performance. This problem also occurs in the antennas described in Japanese Unexamined Patent Application Publication Nos. 10-173425 and 11-312919.
In contrast, according to the multiband antenna described in Japanese Unexamined Patent Application Publication No. 2002-76750, since the LC parallel resonant circuit 111 includes only lumped-constant elements, the loop diameter of the LC parallel resonant circuit 111 is substantially zero. Thus, the LC parallel resonant circuit 111 does not contribute to radiation of electromagnetic waves, and the antenna efficiency is significantly reduced as compared to a situation where an LC parallel resonant circuit is defined by a distributed constant system.